Glass craked on cooling down.

[Laura S]

Hi
I slumped a 6mm thick panel on a batt washed hump mould, that I've used about half a dozen times before. The panel had no visible bubbles and had been properly annealed on it's first firing. It split in two whilst cooling down. I am totally baffled as to why this happened. I used the same schedule that I've used before (175d to 606d 15mins, Full to 482d 1hour, then 83d to 399d) without any problems.  I fired in a Skutt Firebox 14. Any ideas would be much appreciated.

[qwirkyglass]

My gut feeling is thermal shock, how fast did you bring it down at the end?

[silkworm]

I have a Firebox 14 and find that I need to be very careful with slumping anything deeper that the very gentlest of curves. My problem is more often it breaking on the way up rather than down though. I still tend to use the preset programs for slumping and found that by using the slowest one I had less problems, one glass worker I spoke to suggested slowing it even further to heating and cooling at 50 degrees per hour!

[Laura S]

I don't think it can be thermal shock. From the target temperature I cooled on full to 482d , to anneal, and then 83d to 399d. I think there must have been a fault in the glass I wasn't aware of.

[flame n fuse]

you're saying that it split on the way down. Was this because the edges of the break were sharp?

[Laura S]

Yes, the edges were sharp.